When I use this app Net Blocker to block internet for Google Play services, apps can't show ads anymore.
Does AdMob need to load ads via Google Play services? I think that Google Play services should use the network of calling app to request ads.
Because the apps already had the internet access and used AdMob full SDK (not Lite SDK), so I expected that apps can load ads without using Google Play services even when GMS installed on device. Are there any solution in the app side to do that?
the ads come from the Google Play services if installed on the device if you block internet access it will block the process of fetching the ad
If the APK installed on the device is a more recent version, the library will load the classes from the APK service and use these newer classes to request ads. Otherwise, the version of Google Play services that you compile in your app is sufficient to fetch ads.
The benefit here is that you effectively will always run against the latest version of the library when the device has the service installed. And in the case that your service version is older than the version you compiled your app against (ex: your example above) or in the case where there is no service on the device (ex: Kindle Fire or huawei), the library compiled with your app can still do the ad fetching, but you'll still have to pull in updates manually.
